Wireless communication involves transmission of encoded information on a modulated radio frequency (RF) carrier signal. A wireless transceiver includes a power amplifier to amplify a modulated signal to levels sufficient for driving the transmitting antenna. Some wireless communication systems, such as those conforming to the 802.11a standard, can require a high dynamic range for the up-converter and a large power back-off for the power amplifier due to the high peak-to-average power ratios (PAPR) encountered. When high PAPR signals are transmitted through non-linear power amplifiers, the signal will be severely distorted. The high dynamic range requirement and the large power back-off also result in large direct current (dc) power consumption for the transmitter.
Chip producers generally prefer CMOS technology in highly integrated wireless devices because it facilitates fabrication of high-volume, high-complexity, and compatible digital circuitry at low cost. However, CMOS has not typically been a good selection for designers because of its lower performance in terms of linearity and power efficiency. Several classes of CMOS power amplifiers exist, with classes A, AB, and B being most commonly used in wireless communication systems. The three classes of power amplifiers are distinguished by bias conditions, with each class exhibiting unique power gain and efficiency characteristics.
Power gain is defined as the output power divided by the input power, and power efficiency is typically measured as power added efficiency (PAE) defined as the quantity of input power subtracted from the output power, and divided by the DC power consumption. A trade-off exists, particularly in CMOS technology, between power efficiency and linearity. Class A power amplifiers are biased to stay in the saturation region of operation, exhibiting a high linear power gain at low power inputs and low PAE, especially at low power inputs. Class B power amplifiers are biased to shut off the output of a device during approximately half of every cycle. Class AB power amplifiers are biased to shut off the output of the device between 0% and 50% of every cycle. This results in a higher PAE for class B power amplifiers but a lower and more distorted power gain at low power inputs. Class AB power amplifiers have a PAE and power gain linearity that are intermediate between those of class A and class B power amplifiers.